Volcanic ash clouds affecting Northern Europe: the long view     

Graeme T. Swindles  Ivan P. Savov  Charles B. Connor  Jonathan Carrivick  Elizabeth Watson  Ian T. Lawson 《Geology Today》2013,29(6):214-217

The volcanic ash or ‘tephra’ cloud resulting from the relatively small (volume and VEI) eruption of the Icelandic volcano Eyjafjallajökull in 2010 caused major air travel disruption, at substantial global economic cost. On several occasions in the past few centuries, Icelandic eruptions have created ash and/or sulphur dioxide clouds which were detected over Europe (e.g. Hekla in 1947, Askja in 1875, and Laki in 1783). However, these historical observations do not represent a complete record of events serious enough to disrupt aviation in Europe. The only feasible evidence for this is within the geological tephra record. Ash layers are preserved in bogs and lakes where tephra deposited from the atmosphere is incorporated in the peat/mud. In this article we: 1, introduce the analysis of the Northern European sedimentary tephra record; 2, discuss our findings and modelling results; 3, highlight how these were misinterpreted by the popular media; and 4, use this experience to outline several existing problems with current tephra studies and suggest agendas for future research.  相似文献   

Tropical cyclone activity and western North Atlantic stratification over the last millennium: a comparative review with viable connections     

Jonathan D. Woodruff  Ryan L. Sriver  David C. Lund 《第四纪科学杂志》2012,27(4):337-343

Tropical cyclones (TC) are recognized to modify the thermal structure of the upper ocean through the process of vertical mixing. Assessing the role this mixing plays in the overall stratification of the upper ocean is difficult, due to the relatively short and incomplete instrumental record. Proxy records for both TC landfalls and oceanographic stratification are preserved within the geological record and provide insight for how past changes in TC‐induced mixing have potentially affected water column structure prior to the instrumental record. Here we provide the first comparison between previously published paleo‐reconstructions of vertical ocean density and tropical cyclone activity from the western North Atlantic. A prominent lull in TC activity has been observed prior to approximately 1700 CE that extends back several centuries. This interval of low TC activity is shown to be concurrent with the timing of increased ocean stratification near Great Bahama Bank, potentially due in part to reduced TC‐induced mixing. To test whether this relationship is feasible, we present numerical results from a coarse‐resolution ocean general circulation model experiment isolating the effect of TC surface wind forcing on the upper ocean. An anomaly of roughly 0.12 kg m^?3 in vertical stratification occurs above and below the mixed layer for model runs with and without TC mixing. This anomaly is roughly 25% of the entire paleo‐density signal observed just prior to 1700 CE. These results suggest that TC mixing alone cannot completely explain the density anomaly observed prior to 1700 CE, but support TC variability as an important contributor to enhancing oceanic stratification during this interval. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Simulating the response of a closed‐basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi,Ghana)     

Timothy M. Shanahan  Jonathan T. Overpeck  W. E. Sharp  Christopher A. Scholz  Justice A. Arko 《水文研究》2007,21(13):1678-1691

Historical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment‐scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939–2004. Several commonly used models for computing evaporation in data‐sparse regions are compared, including the Penman, the energy budget, and the Priestley–Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy‐budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer‐term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

A synthesis of morphological evolutions and Holocene stratigraphy of a wave-dominated estuary: The Arcachon lagoon,SW France     

Jonathan Allard  Eric Chaumillon  Hugues Féniès 《Continental Shelf Research》2009,29(8):957-969

This work presents the first synthesis of secular to millenary morphological evolutions and stratigraphy of a wave-dominated estuary, the Arcachon lagoon, from a combination of unpublished bathymetric maps (1865 and 2001), core results and high-resolution seismic profiles recorded for the first time in this lagoon. The Arcachon lagoon is located on the Atlantic coast of France, facing the wave-dominated shelf of the Bay of Biscay. It is a mesotidal semi-enclosed environment of about 160 km².The sediment budget of the Arcachon lagoon was computed by subtracting the 1865 bathymetric map from that of 2001. The computed volume difference is low (?9.9±35×10⁶ m³ in 136 yrs) and is the result of the balance between erosion and accretion that occurs within tidal channels and tidal flats, respectively. This morphological evolution pattern is explained by low sediment supply and also by the tidal distortion resulting from the morphology of the lagoon. Deep channels connected to the inlet are dominated by ebb currents inducing erosion. Tidal flats and transverse channels display weak or flood-dominated tidal currents leading to the deposition of silts. The areas of tidal flat siltation locally correlate with the presence of oyster farms, suggesting the influence of Man on the lagoon sediment-fill. Transverse channel-infill is related to weak tidal currents resulting from the hydraulically inefficient orientation of these channels which served as an ancient drainage network.Evidence for tidal channel-infill and channel abandonment are also provided by seismic profiling and cores. The upper stratigraphic succession of the lagoon (about 10 m thick) includes four main stratigraphic units dominated by channel-fills. The two lower units (around 7500–2800 yrs BP) display tabular-shape sandy channels interpreted to be records of the open estuarine phase of the Arcachon lagoon. The two upper units (around 2800 yrs BP to present-day) display U-shaped mixed sand-and-mud channel-fills interpreted to be records of the closure of the lagoon. Given that the basal estuarine units are transgressive and the upper lagoonal units are regressive, the main stratigraphic change at around 2800 yrs BP is interpreted as being the maximum flooding surface (MFS). This late MFS is explained by the low sediment supply. It is proposed that the transition from the estuarine to the lagoonal phase is related to the development of the Cap-Ferret spit in response to an increase in the ratio between wave power to tide power. This change in wave-to-tide ratio may be triggered by wave power increase following the Subboreal/Subatlantic climate instability or a decrease in tide power following a decrease in tidal prism related to the lagoon sediment-fill.Thus, the evolution of the Arcachon lagoon over the last millenaries was mainly controlled by its spit development, leading to a wave-dominated estuary in terms of its geomorphology. Once it was partially closed, extensive mud flats developed in the lagoon which became ebb-dominated.  相似文献   

Hydroecology and ecohydrology: a potential route forward?     

David M. Hannah  Jonathan P. Sadler  Paul J. Wood 《水文研究》2007,21(24):3385-3390

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Equation of state in relativistic magnetohydrodynamics: variable versus constant adiabatic index     

A. Mignone  Jonathan C. McKinney 《Monthly notices of the Royal Astronomical Society》2007,378(3):1118-1130

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Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC     

Alexandre Faure  Hemal N. Varambhia  Thierry Stoecklin  Jonathan Tennyson 《Monthly notices of the Royal Astronomical Society》2007,382(2):840-848

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Disc–jet coupling in black hole accretion systems – I. General relativistic magnetohydrodynamical models     

Jonathan C. McKinney  Ramesh Narayan 《Monthly notices of the Royal Astronomical Society》2007,375(2):513-530

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The Monitor project: rotation of low-mass stars in the open cluster NGC 2516     

Jonathan Irwin  Simon Hodgkin  Suzanne Aigrain  Leslie Hebb  Jerome Bouvier  Cathie Clarke  Estelle Moraux  D. M. Bramich 《Monthly notices of the Royal Astronomical Society》2007,377(2):741-758

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The Monitor project: JW 380 – a 0.26-, 0.15-M_⊙, pre-main-sequence eclipsing binary in the Orion nebula cluster     

Jonathan Irwin  Suzanne Aigrain  Simon Hodgkin  Keivan G. Stassun  Leslie Hebb  Mike Irwin  Estelle Moraux  Jerome Bouvier  Aude Alapini  Richard Alexander  D. M. Bramich  Jon Holtzman  Eduardo L. Martín  Mark J. McCaughrean  Frédéric Pont  P. E. Verrier  María Rosa Zapatero Osorio 《Monthly notices of the Royal Astronomical Society》2007,380(2):541-550

We present a general recipe for constructing N -body realizations of galaxies comprising near spherical and disc components. First, an exact spherical distribution function for the spheroids (halo and bulge) is determined, such that it is in equilibrium with the gravitational monopole of the disc components. Second, an N -body realization of this model is adapted to the full disc potential by growing the latter adiabatically from its monopole. Finally, the disc is sampled with particles drawn from an appropriate distribution function, avoiding local-Maxwellian approximations. We performed test simulations and find that the halo and bulge radial density profile very closely match their target model, while they become slightly oblate due to the added disc gravity. Our findings suggest that vertical thickening of the initially thin disc is caused predominantly by spiral and bar instabilities, which also result in a radial re-distribution of matter, rather than scattering off interloping massive halo particles.  相似文献